Use shielded twisted-pair cable for signals, ground the shield at one end, keep signal cables away from power and motor cables, and avoid ground loops by giving the loop a single reference. Noisy readings that follow pump or motor starts usually trace to shared grounds, unshielded cable or cables run beside power. A clean installation starts with the layout of the cable routes.
- Choose cables for power, 4-20 mA, relays and Modbus
- Ground shields correctly and avoid ground loops
- Diagnose noise that follows plant events
Cable choices
| Circuit | Cable |
|---|---|
| Mains power | Power cable sized for the load, with protective earth |
| 4-20 mA | Shielded twisted pair, instrument cable |
| Modbus RS-485 | Shielded twisted pair with the right impedance, terminated at both ends |
| Relay outputs | Cable rated for the switched voltage; separate from low-level signals |
Run each type in its own tray or conduit where possible.
Shield and ground
A shield protects the signal by carrying interference to ground. If it is grounded at both ends in a plant with different ground potentials, current flows through the shield and injects noise: a ground loop. The common practice for instrument shields is to ground at one end only, usually at the control-system end, and leave the other end insulated. Follow the standard of your plant and the manufacturer's instructions.
Grounding the shield at both ends and also at several junction boxes. Each extra ground makes a new loop.
Separation from power
- Keep signal cables at least a cable tray's width from motors, drives, contactors and power cables, and cross them at right angles where they must cross.
- Use separate trays for different voltage classes.
- Avoid running signal cables beside variable speed drives, which are strong noise sources.
Surge, lightning and isolation
Long cables outdoors pick up surges. Surge protectors on field cables and galvanic isolators on loops protect analyser electronics and break ground loops. Install them where the cable enters the building or cabinet.
Chasing noise
- Note when the noise happens: with a pump start, a drive change, a nearby radio?
- Check shield grounding and any duplicate grounds.
- Check cable segregation and routing near power.
- Disconnect the signal and read the analyser locally: if local reading is quiet, the noise is in the wiring.
- Add isolation or filtering as a last step.
Remember that an apparent electrical fault can be a sample fault, such as pulsing flow: Sample system troubleshooting: symptoms to causes.
- Use shielded twisted pair for signals and keep it apart from power.
- Ground shields at one end to avoid ground loops.
- Surge protection and isolation protect long outdoor runs.
- Time-match noise to plant events to find its source.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
Why ground an instrument shield at one end only?
Grounding at both ends can create a ground loop that carries current and injects noise.
Noise appears every time a large pump starts. What do you check?
Cable segregation from the pump's power cable, shield grounding and shared grounds.
What is a ground loop?
A circuit formed by two or more grounding points at different potentials, which drives unwanted current through a signal path.
Key terms
Frequently asked questions
How far apart should power and signal cables run?
Use separate trays; a few tens of centimetres of separation is common, and follow your plant's standard.
Can I run Modbus beside power cables?
Avoid it; use shielded twisted pair and keep it separate.
What causes a noisy 4-20 mA signal?
Ground loops, unshielded cable, cables beside power, or a loose terminal.
Do I need surge protection?
For long outdoor cables, yes.
